<p>Four typical admixtures, polycarboxylate superplasticiser (PCE), tartaric acid (TA), sorbitol and polyacrylamide (PAM), were selected to systematically investigate their regulatory mechanisms on the formation of ettringite through Fourier transform infrared spectroscopy, X-ray diffraction, particle size analysis and nucleation kinetic model. The experimental results indicate that the admixtures alter the formation of ettringite through physical adsorption, complexation and solution viscosity modulation without changing its chemical structure. Low concentrations of PCE inhibit size growth by forming an adsorption layer on the surface of ettringite, whereas high concentrations of PCE alter the size change of ettringite by modulating the distribution of ionic concentrations. TA significantly reduces the size of ettringite by complexing Ca²⁺. Sorbitol and PAM promote the local growth of ettringite at low concentrations, leading to larger sizes. But at high concentrations, the size growth of ettringite is inhibited due to the increase in viscosity or the enhancement of complexation. Matlab nucleation kinetic modelling further shows that the addition of admixtures enhances the initial nucleation during ettringite synthesis, with values ranging from 14.45% to 114.25%. However, the subsequent nucleation rate of ettringite is significantly affected, decreasing by 12.79% to 71.74%. The results provide a theoretical basis for the design of ettringite materials and the optimisation of the application of admixtures.</p>